TEM observation of moire´fringe in the HgBa 2Ca 2Cu 3O 8+δ superconductor

Abstract

The intergrowth structure in a nearly single-phase polycrystalline sample of HgBa 2Ca 2Cu 3O 8+δ has been investigated in detail by using X-ray diffraction, scanning electron microscopy(SEM) and transmission electron microscopy (TEM). Stacking faults in our sample of Hg-1223 were observed under TEM. It was clearly shown that the stacking fault was formed by appropriate combination of Hg-1212/Hg-1223 and Hg-1234/Hg-1223 intergrowth structures. The congruence relationship for these intergrowth structure are described respectively by { 00 1 } H g − 1212 / / { 00 1 } H g − 1223 , 〈 11 0 〉 H g − 1212 / / 〈 11 0 − 〉 H g − 1223 ; a n d { 00 1 } H g 〉 1234 / / { 00 1 } H g − 1223 , 〈 11 0 〉 H g − 1234 / / 〈 11 0 〉 H g − 1223. . At the conjunction boundary, some lattice strains always exist. However, for a stable defect structure the strain energy strored in the conjunction boundary should be minimized by reduction in strain. Thus the intergrowth phase of Hg-1212 or Hg-1234 will be gradually annihilated. As aconsequence, a small twistangle will be resulted between the matrix and the intergrowth phase. The occurrence of this small orientation difference will form a Moire´fringe under appropriate diffraction condition. This indicates that the mercury-based copper oxide superconductor has a crystal structure with layer-type characteristic.